1. Field of the Invention
The present invention relates generally to telephone data transmission systems, and more particularly to a time synchronized data transmission system which allows a central computer, for example, to access a remote data set, such as an electronic publication storage device, a hybrid telephone-cable TV polling system, or a computer data base subscriber's data set, via telephone lines and during predetermined time intervals, and communicate with that data set without at the same time ringing the subscriber's telephone. The present invention finds application in many areas, one of which, for instance, might be a shop-by-computer system where it might be necessary to transmit information to and from an individual subscriber's data set connected to an information storage device. Another might be the field of power demand data retrieval, wherein a central computer accesses a remote data set which has stored therein data corresponding to the electrical or other power consumed at the remote location. Still another might be remote control of heating, ventilating and air conditioning equipment through telephone lines.
2. Description of the Prior Art
Various systems are presently in existence which allow central computers to access individual subscriber's data sets via dedicated telephone lines. It is preferable to use the subscriber's existing telephone line rather than install a separate dedicated line to provide access to the data set. Thus, systems of this type require that the telephone line be available for both use by ordinary persons and by computers calling the subscriber. A requirement of systems of this type is that when computer calls are received the call is automatically transferred to a data set and the telephone does not ring.
In applicant's prior U.S. Pat. No. 4,345,113, an automatic telephone message interception system is described which permits the use of a single telephone line for both ordinary personal use and for receiving calls from a computer, wherein computer calls are automatically transferred to the subscriber's data set without ringing the subscriber's telephone. An ordinary (non-computer) call made to the subscriber, would, however, be answered by the automatic telephone message interception system described in the above U.S. patent even if no one were present at the subscriber's telephone. The caller would thus be charged for a brief call even though no one answered the phone.
The subscriber could, of course, solve this problem by shutting off the automatic system if the telephone is to be left unattended, but the data set could not be accessed by the computer while the system was shut off.
Thus, the need exists for a system which both allows a central computer to access the subscriber's data set via a non-dedicated telephone line and which allows normal operation of the telephone by both the subscriber and other persons calling the subscriber. In particular such a system should allow a computer to access the subscriber's data set without ringing the subscriber's telephone and should allow persons to call the subscriber at any time without being charged for the call if the telephone is unanswered.